1. Field of the Invention
The invention is generally related to compositions for reducing wear of rubbing surfaces, wherein the compositions include a combination of a cyclic amide and a monoester formed by reacting a dimer acid with a polyol.
2. Description of the Prior Art
Wear has been defined as the progressive loss of a substance from the operating surface of a body as a result of relative motion at the surface of the body (see, Furey, "Tribology", Encyclopedia of Materials Science & Engineering, Pergamon Press, Oxford, pp. 5145-5157, 1986). When elements rub together, whether made of the same or different materials, wear can occur. The rate of wear tends to increase under harsh temperature and pressure conditions which, for example, exist inside ceramic or metal engines, propulsion engines, and the like. In addition to limiting the useful life of the part in which the ceramic or metal is used, wear of ceramics or metal can be costly because the ceramic or metals materials themselves are expensive to produce. Other significant problems associated with wear include, e.g., down time for equipment, reduced safety, and diminished reliability.
Therefore, lubrication, particularly under boundary friction conditions, is extremely important for rubbing materials. Lubrication is a process that reduces friction and/or wear (or other forms of surface damage) between relatively moving surfaces by the application of a solid, liquid, or gaseous substance (i.e., a lubricant). Therefore, the primary function of a lubricant is to reduce friction or wear or both between moving surfaces in contact. However, lubricants can also serve other ancillary functions, such as acting as a hydraulic fluid, coolant, gas seal and carrier for adhesives; they may also protect metal surfaces from corrosion and aid in the removal of debris and deposits. Examples of conventional lubricants are widespread and diverse. They include automotive engine oils, wheel bearing greases, transmission fluids, electrical contact lubricants, rolling oils, cutting fluids, preservative oils, gear oils, jet fuels, instrument oils, turbine oils, textile lubricants, machine oils, jet engine lubricants, air, water, molten glass, liquid metals, oxide films, talcum powder, graphite, molybdenum disulfide, waxes, soaps, polymers, and even the synovial fluid in human joints.
For instance, in the manufacture of small 4-stroke engines, such as used in lawn care equipment, it is customary to precoat certain parts (e.g., piston rings, cylinder, crankshaft bearings, cams) with special oils or greases prior to assembly, and then to carry out a short-time "hot test" of the engine using a normal charge of oil added to the crankcase. After the test, the normal charge of oil is then drained out using a suction device. However, some residual oil tends to remain in the engine. This represents not only an economic loss in terms of material and labor costs since large numbers of engines are involved, but also poses a possible leakage problem during shipping or upon first use in a particular application.
U.S. Pat. No. 3,377,285 to Randles teaches a nonthickening oil concentrate in which mineral oil additives containing an oil soluble ester copolymer are inhibited from increasing in viscosity or gelling by addition of a minor amount of a non-polymerizable nitrogen-containing heterocyclic compound having the ##STR1## unit in the molecule.
U.S. Pat. No. 3,180,832 to Furey teaches lubricity and antiwear additives involving ester reaction products of substantially equimolar quantities of oil-soluble dimer acids with diols or polyols.
More recently, the environments where lubrication needs arise continue to evolve. For instance, in machinery, the classical lubricants and additives more typically have addressed applications involving rubbing parts made of metal, in particular, steel or its alloys. However, more recently there also has been increased interest in using ceramic materials and fiber-reinforced plastics (i.e., composites) in a wide variety of applications which traditionally have utilized metals. Ceramic and composite materials have several advantageous engineering properties. For example, ceramics generally can be used at much higher temperatures than metals, are relatively inert and resist corrosion, and are resistant to abrasive wear owing to their hardness. Additionally, some ceramics are lighter in weight than conventional steel-based materials. Alumina, silicon nitride, partially stabilized zirconia, and silicon carbide, for example, are ceramic materials being used in high temperature wear environments.
Ceramics thus have attracted increased interest for uses along side, in combination with, and/or in lieu of metals, such as in automotive engines, gas turbines, turbomachinery, cutting tools for super alloys, and aerospace bearings, which are driven by a need for industrial materials that can tolerate high temperature, corrosive environments and/or result in greater efficiency. However, the surface characteristics of ceramics are very different from those of metals. For these and other reasons, conventional metal lubricants generally have lacked the versatility for successful use in the lubrication of ceramics.